This invention relates to a sound damping filter for an ear protector placeable in the auditory duct of a person, comprising a sound damping canal with a passage extending from a sound inlet opening to a sound outlet opening.
Such a filter is known from practice and is utilized in a housing placeable in the auditory duct of a person in order to damp the intensity of sound to be supplied to the eardrum of the person. By varying the cross section and the length of the sound damping canal, the level of damping can be influenced, and a sound damping filter can be manufactured which is tailored to a particular application.
A disadvantage of the known filter is that the damping action of the damping canal is always stronger for higher sound frequencies than for lower frequencies. Consequently, the known filter has a damping characteristic rising, for instance, from 20 dB at 125 Hz to 40 dB at 8,000 Hz. A person wearing ear protectors in his ears which are tuned to damping machine noise of a frequency of about 50-500 Hz will perceive sound of a higher frequency, for instance the higher speech frequencies between 1 kHz and 3 kHz, only relatively weakly. Not only does this create the impression for the person that he is closed off from the environment, but also dangerous situations may arise in that the person perceives higher frequencies, for instance warning signals, unduly damped.
Also known are filters for ear protectors where the passage of the damping canal is closed off by means of a membrane, such that, during wearing, between the membrane and the eardrum of the person an amount of air is locked. Although an ear protector with such a filter enables a damping level that remains substantially the same over the audiofrequency range, there are disadvantages associated with this type of ear protector as well. In particular, the locked air volume has as a consequence that a wearer, due to conduction of the bones around the ear, has an enhanced perception of swallowing and chewing sounds, due to the so-called occlusion effect. In addition, the closure may give rise to transpiration in the auditory duct.
The object of the invention is to provide a sound damping filter of the type mentioned in the opening paragraph hereof which does not possess the above-mentioned disadvantages. To that end, a sound damping filter according to the invention comprises a sound damping canal, the passage of which is closed off by means of an air permeable, sheetlike membrane, such that transport of air through the filter is possible. What is thus achieved is that a filter can be composed having a flatter damping characteristic, while through transport of air the occurrence of the occlusion effect and transpiration can be avoided.
The damping canal can then be chosen to have so large a diameter and/or so short a length that an increase of the damping of high frequencies is relatively small, whilst the membrane takes care of sufficient damping of low frequencies.
By adjustment of the geometry of the damping canal, the damping of high frequencies can be adjusted, without the damping for the low frequencies thereby being materially influenced. By adjustment of the geometry of the membrane and of the air permeability of the membrane, the damping of low frequencies can be set, without the damping for the high frequencies thereby being materially influenced.
Although the air permeability of the membrane can already be realized by the use of an air-permeable porous material which is provided with micropores, it is presently preferred to manufacture the membrane of substantially airtight material in which at least one perforation is provided. By increasing the number of perforations and/or the diameter thereof, the damping of low frequencies by the membrane can be reduced. By providing perforations of substantially equal diameter, the low tone damping can simply and reproducibly be stepwise influenced. What is achieved by choosing the diameter of the perforations to be substantially equal to the thickness of the membrane, is that low tone damping in practice can be set in a simple manner. What is achieved by the use of a flexible membrane is that in particular for higher frequencies a lower damping can be realized.
It is noted that when in this context reference is made to low frequencies, these are understood to include in any case frequencies lower than 1,000 Hz, while higher frequencies are understood to comprise at least frequencies higher than 1,000 Hz.
It is additionally noted that in this context a sheetlike membrane is understood to mean at least a membrane with a nonwoven structure.
Further advantageous embodiments of the invention are described in the dependent claims.
The invention further relates to an ear protector and to a method for manufacturing a membrane for a filter for an ear protector.
The invention will now be further elucidated on the basis of an exemplary embodiment represented in a drawing. In the drawing: